Relationships between serosal medium potassium concentration and sodium transport in toad urinary bladder. I. Effects of different medium potassium concentrations on electrical parameters.

When serosal medium potassium was decreased from the usual concentration of 3.5 mM, the short-circuit current (SCC) of hemibladders in chambers immediately and transiently increased. The maximum SCC attained was greater the greater the decrease in serosal potassium, and was twice the initial SCC when the final serosal medium was potassium-free. The SCC then fell to its previous level for final serosal potassium concentrations greater than 2 mM and to less than its previous level for those less than 2 mM, being lowest (15% of previous levle) in potassium-free sodium Ringer's. When serosal medium potassium was increased from 3.5 mM by substituting potassium for sodium, SCC transiently decreased and then recovered to its previous level. Steady SCC was the same in serosal media of 2-116 mM potassium; conductance increased and p.d. decreased after incubation in 50-116 mM potassium serosal media. Short-circuit current and p.d. transiently increased (decreased) whenever serosal medium potassium was decreased (increased); conductance increased with any change in serosal potassium. Changing mucosal medium potassium concentration between 0 and 50 mM did not affect SCC. The initial transient increase and subsequent decrease in SCC on removing serosal potassium were partially prevented by 3.5 mM rubidium or caesium, or by 116 mM choline in the serosal medium. The transient changes in SCC were due partly to changes in transepithelial sodium transport.